That is not an easy question. Let me say at the outset that maybe none of them do, in the sense of having broad applicability. However, the use of kinetic isotopes to study the transition-states of enzyme-catalyzed reactions is pretty amazing, and sometimes these isotope effects are used to aid in inhibitor design (the work of Vern Schramm comes to mind). Some of the best enzymologists of the twentieth century (Frank Westheimer, William P. Jencks, and Robert Abeles, for example) worked on many problems, and although their contributions were large, they were spread out.
EDT
The notion that enzymes could work by radical mechanisms was also a conceptual breakthrough, IMO. Ribonucleotide reductase is a good example.